The rates of cementation, i.e., the conditions for obtaining the profile of carbon concentration in the shortest possible time, depend upon the rates of transfer of the chemical species in the liquid phase or in the gaseous phase (e.g., carbon monoxide in the gaseous phase), the rates of transfer of the carbon at the liquid-solid or gas-solid interface, and the diffusion of carbon in the solid phase.
In the conventional processes, the limiting stage is that corresponding to the transfer of the carbon to the interface, either because the decomposition reaction of the chemical species is slow or because the decomposition products of these species cause a resistance to the transfer of the carbon at the surface of the steel.
Thus, for the conventional processes of case-hardening in the gaseous phase by carbon monoxide, the elimination of the oxygen is too low, which greatly reduces the flux of carbon transferred. This is shown in that the surface concentration of carbon does not reach the maximum value, permitting the diffusion of the carbon into the solid at the optimum rate, until quite some time, e.g., 20 min., after having been put in contact with the appropriate atmosphere in the usual conditions of temperature and pressure.